1. Field of the Invention
The present invention relates to a scandia-stabilized zirconia electrolyte for a solid oxide fuel cell and, more particularly, to a scandia-stabilized zirconia electrolyte for a solid oxide fuel cell, which is configured such that at least one oxide selected from among gadolinium oxide (Gd2O3) and samarium oxide (Sm2O3) is co-doped with ytterbium oxide (Yb2O3) to thus improve stability in a reducing atmosphere.
2. Description of the Related Art
Among a variety of fuel cells, a solid oxide fuel cell (SOFC) using, as a key part, a ceramic unit cell comprising ceramic electrodes and an electrolyte may operate at the highest temperature, thus exhibiting very high energy conversion efficiency. Furthermore, SOFC enables a gas turbine or a micro gas turbine to operate in two steps using high-temperature steam that is discharged through high-temperature operation, thus favorably constructing a high-efficiency hybrid power generation system. In addition, since SOFC advantageously has high fuel selectivity, not only hydrogen fuel gas but also various kinds of hydrocarbonaceous fuel or bio gas may be used. Also, the U.S. Department of Energy (DOE) aims to develop MW-class integrated gasification fuel cell (IGFC) technology.
Scandia-stabilized zirconia (Sc2O3-stabilized ZrO2, abbreviated as “ScSZ”) is a zirconia-based electrolyte material having the highest oxygen ionic conductivity due to the use of Sc3+ (0.87 Å) having a cation radius that approximates the cation radius (0.84 Å) of Zr4+.
In particular, (Sc2O3)0.11(ZrO2)0.89 (abbreviated as “11ScSZ”) is regarded as the ideal electrolyte material because its ionic conductivity is not lowered even after long-term use in an air atmosphere, but the 11ScSZ electrolyte material has been found to suffer from phase transition, showing a monoclinic structure at a temperature lower than about 630° C. and a cubic structure at higher temperatures.
With the goal of solving such a phase transition problem, thorough research and development is ongoing. In this regard, Toho Gas in Japan has developed and commercialized a novel electrolyte that is stabilized into a cubic structure in the temperature range from room temperature to high temperatures by substituting some elements of the 11ScSZ electrolyte with ceria (CeO2). As disclosed in JP 2008-305804 A, the electrolyte composition of Toho Gas is configured such that 8.5 to 15 mol % of scandia is doped with 0.5 to 2.5 mol % of yttria and/or ceria and simultaneously the total amount of scandia and yttria and/or ceria is set in the range of 9 to 15 mol %. An actually commercially available electrolyte product is 10Sc1CeSZ (10 mol % Sc2O3-1 mol % CeO2-89 mol % ZrO2).
The 10Sc1CeSZ electrolyte has solved the phase transition problem, but new problems occur therein. Specifically, a unit cell using the 10Sc1CeSZ electrolyte is problematic because the power density (W/cm2) is continuously decreased with an increase in the operating time thereof.
This is considered to be due to the electrolyte material. Specifically, in the case where ceria (CeO2), used for 10Sc1CeSZ, is placed in a reducing atmosphere, Ce4+ may be converted into Ce3+ at a temperature equal to or less than 650° C. In real-world applications, the 10Sc1CeSZ electrolyte shows a color close to a white color after sintering and maintains the same color in a high-temperature air atmosphere, but the color is converted into an orange color in a high-temperature reducing atmosphere.
Hence, the instability of the 10Sc1CeSZ electrolyte in a reducing atmosphere is deemed to cause the continuous power decrease in the unit cell, and thus, there is a need to develop a novel zirconia electrolyte material in which the scandia-stabilized zirconia is stabilized into a cubic crystal structure and the stability thereof is improved in a reducing atmosphere.